With the successful installation of a composite arch highway bridge near Duvall, Wash. – the first of its kind on the West Coast – the Washington State Department of Transportation (WSDOT) has gained a cost-effective and sustainable alternative to traditional bridge construction.

It comes at a critical time. A federal injunction requires WSDOT to rebuild approximately 400 fish barriers under state highways in the northwest part of the state by 2030. The work will replace existing culverts beneath roadways and bridges and restore the natural stream habitats so the water flows freely and salmon, coho, chinook and steelheads can easily swim through.

Many of the fish barriers will feature buried structures. WSDOT typically uses split box culverts made from pre-cast concrete when buried structures are needed, but manufacturing the required number of pre-cast components would strain the capacity of the state’s producers. The department considered other options and chose a composite structure from AIT Bridges, a division of Advanced Infrastructure Technology. The company uses the composite arch bridge technology originally developed for the military at the University of Maine’s Advanced Structures and Composites Center. AIT Bridges has also developed GFRP deck panels that are placed across the arches.

AIT Bridges produces the hollow, tubular bridge arches (GArches) and GFRP decking (GDeck) at its factory in Brewer, Maine. Once installed on site, the arches are covered with decking and then filled with reinforcing concrete. Since 2008, the company has installed 30 GArch composite bridge systems, most on the East Coast.

The 50-foot-long bridge in Duvall is located on state Route 203 over Loutsis Creek. Since the site is steep, building a pre-cast bridge would involve renting and transporting large cranes to lift the pieces into place. For the composite components, however, general contractor Goodfellow Brothers needed only a forklift and straps. “That probably saved some money, because the pre-mobilizations to set big pieces of concrete are really expensive operations,” says Mark Gaines, WSDOT bridge and structures engineer.

Another benefit of composite bridge structures is their longevity and low lifecycle cost. “That’s a big sustainability issue. If you can have 100-plus years life and still have very minimal maintenance, that’s a win/win,” says Ken Sweeney, AIT Bridges’ group president and chief engineer. Over the long term, concrete bridges crumble and their steel reinforcements rust, eventually polluting the waters below. That doesn’t happen with composites. In addition, it takes less material to build composite bridges than concrete pre-cast structures.

Before awarding the sole source contract to AIT Bridges, WSDOT carefully reviewed all the engineering data about the composite arch bridge’s ability to withstand fire and damage caused by objects like floating logs striking it. Earthquakes were another concern. “This is the first time I’m aware of that [AIT] used the composite arch in a high-seismic zone, so we wanted to make sure that it met our seismic design requirements,” Gaines says. “We asked lots of difficult questions. But in the end, they had answers to everything we asked, and we were quite comfortable moving forward.”